George Lucas didn’t “get involved,” and multiple endings weren’t shot. Multiple versions of the same ending were shot, (multiple takes, really. Again - just normal everyday moviemaking stuff) but the ending was always essentially the same. It’s a thing with Star Wars - a ton of the trivia everyone “knows” about Star Wars is basically just guesswork and rumor repeated enough that everyone accepts it because it’s easier to win online arguments that way.
The multiple endings thing especially - that was some guy on Twitter incorrectly reporting a story he saw via some other guy on Twitter, IIRC. Or Reddit, probably. Part of why it’s so easy for all this to become a YouTube scam for ad money is because even the “upstanding insiders” or whatever are just faking-it-til-they-make-it. People want to believe they know something other people don’t, it makes all the time spent chasing this stuff feel sort of worthwile, and at that point it’s just a matter of figuring out which flavor of fandom you prefer, the angry stuff or the blindly hopeful stuff. If you’re starting with a story pre-tainted with misinformation because the people volunteering for their insider “job” - however well-intentioned they might be - don’t know how to do it, it’s really easy to then further twist that misinformation to fit your chosen grift. Once everyone involved tacitly agrees facts don’t really matter, the game is on!
Knowing what actually happened is almost always secondary to attaining the feeling of knowing what happened. This isn’t just a Star Wars thing, either. But as with all things fandom, you see it in Star Wars “communities” because that same phenomenon happens in places where it’s actually important, too. This is just the safer, less consequential version of it.
I’m just as skeptical as you, but my point is that ZSJL was the game changer. The only difference here is that JJ was not fired and didn’t leave the project. So unlike Snyder, JJ had to agree with the final theatrical version. He would have to publicly admit that an alternate cut EXISTS like Snyder did. Either way, I don’t doubt there could be a longer version that had to be edited down for a theatrical release. Almost every film has this, but it’s only in demand when the theatrical version is unsatisfying to the audience or filmmaker. And there certainly is deleted footage and most likely alternate footage. Does it make the movie better? I doubt it. But that footage is invaluable to fan editors.